The present invention relates to a system architecture for scan line non-linearity in a Raster Output Scanning (ROS) system and, more particularly, to a system architecture that enables correction of scan non-linearity and “mix and match” of ROS units and pixel boards at random during the manufacture of printing systems.
Printing systems utilizing lasers to reproduce information are well known in the art. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of a photosensitive medium, such as a photoreceptor, to record an electrostatic latent image on the photosensitive medium.
A plurality of ROS units can be used in a color xerographic ROS printer. Each ROS forms a scan line for a separate color image on a common photoreceptor belt. Each color image is developed in overlying registration with the other color images from the other ROS units to form a composite color image that is transferred to an output sheet. Registration of each scan line of the plurality of ROS units requires each image to be registered to within a 0.1 mm circle or within a tolerance of ±0.05 mm.
A typical prior art raster output scanning system 10 of FIG. 1 includes a light source 12 for generating a light beam 14 and scanning means 16 for directing the light beam 14 to a spot 18 at a photosensitive medium 20. The scanning means 16 also serves to move the spot 18 along a scan line 22 of specified length at the photosensitive medium 20. For that purpose, the scanning means 16 in the illustrated scanner system 10 includes a rotatable polygon mirror with a plurality of light reflecting facets 24 (eight facets being illustrated) and other known mechanical components that are depicted in FIG.1 by the polygon 16 rotating about a rotational axis 26 in the direction of an arrow 28.
The light source, 12, such as a laser diode, emits a modulated coherent light beam 14 of a single wavelength. The light beam 14 is modulated in conformance with the image information data stream contained in the video signal sent from image output light source control circuit 30 to the light source 12.
The modulated light beam 14 is collimated by a collimating lens 32, then focused by a cross-scan cylindrical lens 34 to form a line on a reflective facet 24 of the rotating polygon mirror 16.
The polygon mirror 16 is rotated around its axis of rotation by a conventional motor.(not shown), known to those of ordinary skill in the art.
The beam 14 reflected from the facet 24 then passes through the f-theta scan lenses 36 and the anamorphic wobble correction lens 38.
The f-theta scan lens 36 consists of a negative plano-spherical lens 40, a positive plano-spherical lens 42, and the cross-scan cylinder lens 44. This configuration of f-theta scan lenses has sufficient negative distortion to produce a linear scan beam. The light beam will be deflected at a constant angular velocity from the rotating mirror that the f-theta scan lens optically modifies to scan the surface at a nearly constant linear velocity.
The f-theta scan lens 36 will focus the light beam 14 in the scan plane onto the scan line 22 on the photosensitive medium 20.
After passing through the f-theta scan lens 36, the light beam 14 then passes through a wobble correction anamorphic lens element 38. The wobble correction optical element can be a lens or a mirror and is sometimes referred to as the “motion compensating optics”. The purpose of optical element 38 is to correct wobble along the scan line generated by inaccuracies in the polygon mirror/motor assembly.
The wobble correction lens 38 focuses the light beam in the cross-scan plane onto the scan line 22 on the photosensitive medium 20.
As the polygon 16 rotates, the light beam 14 is reflected by the facets 24 through the f-theta and wobble correction lenses and scans across the surface of the photosensitive medium in a known manner along the scan line 22 from a first end 46 of the scan line 22 (Start of Scan or “SOS”) past a center (the illustrated position of the spot 18) and on to a second end 48 of the scan line 22 (End of Scan or “EOS”). The light beam exposes an electrostatic latent image on the photosensitive member 20. As the polygon 16 rotates, the exposing light beam 14 is modulated by circuit 30 to produce individual bursts of light that expose a line of individual pixels, or spots 18, on the photosensitive medium 20.
Ideally, the ROS should be capable of exposing a line of evenly spaced, identical pixels on the photosensitive medium 20. However, because of the inherent geometry of the optical system of the ROS, and because manufacturing errors can cause imperfections in the scan optics, obtaining evenly spaced, identical pixels can be problematic.
“Scan non-linearity” refers to variations in pixel placement relative to uniform pixel placement, and this is primarily due to variation of the spot velocity occurring as the spot moves along the scan line during the scan cycle relative. Scan linearity is the measure of how equally spaced the spots are written in the scan direction across the entire scanline. In order to define scan linearity spatially, reference points on the scan line must first be specified: one definition is to consider that the two idealized ends of the scan line have zero error, provided that the delay from SOS to the first active is adjusted correctly (correct margin adjustment) and provided the last pixel of the active scanline is also adjusted correctly (correct magnification adjustment). Active scan is defined as that part of the scan line, which is modulated by the video stream. With the start of active scan and the end of active scan defined as reference points, typical scan linearity curves start at zero position error at one end of the active scan and end with zero position error at the other end of active scan. In between the endpoints of the active scan the non-linearity curve can have a multitude of shapes. Ideally, the curve of non-linearity versus scan distance would be at zero across the entire scanline for perfect pixel placement. From optical modeling for one lens design, the ideal shape would appear as in FIG. 4. In practice the shape may have only one lobe (above or below) the zero non- linearity line, or non-symmetrical lobes that are distorted from the sinusoidal-like appearance and the number of zero crossings of non-linearity of the ROS can vary from unit to unit.
Scan non-linearity is typically caused by system geometry or a velocity variation of the scanning means. The speed at which the focussed exposing light beam travels across the scan line on the photosensitive medium 20 is called the spot velocity.
Without some means to correct for the inherent scan non-linearity caused by the geometry of the ROS system, the spot velocity will vary as the light beam scans across the photosensitive medium. The video input of some raster output scanners compensate for such non-linearity electronically using a variable frequency pixel clock (sometimes called a scanning clock). The pixel clock produces a pulse train (i.e., a pixel clock signal) that is used to turn the light beam emitted by the light source on and off at each pixel position along the scan line. By varying the clock frequency and thereby the timing of individual pulses in the pulse train serves to control pixel placement along the scan line. On the one hand, if the frequency of the pixel clock signal is constant, the resulting pixels will be advanced or retarded relative to the uniform pixel placement because of scan nonlinearity of the ROS. On the other hand, if a controlled variation of the pixel clock is properly adjusted across the scan, the pixels can be place with minimal deviation from the ideal of equally space pixels. That will more evenly space the pixels and thereby at least partially compensate for what is sometimes called pixel position distortion (i.e., uneven pixel spacing caused by scanner non-linearity).
The pixel clock control circuitry 30 serves as an electronic control system for synchronizing the light beam 14 modulation in order to produce the pixels along the scan line 22. The control system that includes video and pixel clock may, for example, be configured using known components and design techniques to produce a control signal for activating the light beam at each of a plurality of desired pixel positions along the scan line (e.g., the central portion of each pixel position being evenly spaced at 1/300 inch intervals for 300 dpi resolution or being evenly spaced at 1/600 inch intervals for 600 dpi resolution, etcetera).
Preferably, the control system is configured so that the control signal defines a pixel interval for each pixel position and so that the pixel interval defined by the control signal varies according to spot velocity, to correct for scan non-linearity as described above. The control system may also synchronize the control signal with spot position by suitable known means, such as by responding to a start-of-scan (SOS) control signal or other synchronizing signal produced by known means, in order to enable alignment of the start of the sequence of pixels in each scanline. This sequence of pixels is delayed by a defined clock count in order to set the edge margin of the active scan scanline. The average clock frequency can also be adjusted to change the scan magnification. If the average clock frequency is increased, the scan magnification is decreased and conversely if the average clock frequency is decreased the scan magnification is increased. The settings of margin and magnification are sensed by marks on the photoreceptor as shown by block 118 in FIG. 7.
FIG. 2 shows a an ideal scan line 100 consisting of a series of pixels 102 uniformly spaced 104 by the pixel clock of the raster output scanning system. These pixels 102 on the scan line 100 are placed on a uniform grid 106 at each clock cycle in the idealized case of perfect scan non-linearity.
FIG. 3 illustrates deviation from the uniform pixel placement of FIG. 2 due to scan non-linearity. The scan line 200 consists of a series of pixels 202 which are displaced by a distance 204 from the uniform pixel placement 206 along the scan line as shown schematically in the graph of FIG. 4.
For color printing system with multiple ROS units the accurate registration of pixels in the fast scan direction is required. The ideal is to place the pixels from each ROS along a uniform grid when the pixel clock frequency is constant. In practice there is a departure from the uniform grid, called scan non-linearity, and this non-linearity profile varies from unit to unit.
Pixel clock frequency variation is used to compensate for this profile. A lookup table is used to control the pixel clock frequency variation, according to a prescribed algorithm. The transfer function of clock frequency change vs. table value also varies among pixel boards. The present invention is a non-linearity correction-system architecture that enables the “mix and match” of a ROS and a pixel board taken at random during the printing system manufacture.